There has been a continuous need for providing high temperature operation in regard to pressure transducers. Such pressure transducers capable of operating at high temperatures while further being of small physical dimensions are extremely desirable for use in many areas. Cognizant of the problem the assignee herein, Kulite Semiconductor Products Inc., has developed and fabricated high temperature transducers from silicon carbide (SiC). Silicon carbide, as one can ascertain, can operate at temperatures well above 600° C. where conventional transducers, such as silicon devices undergo plastic deformation at such temperatures, rendering the devices relatively useless. Thus, the use of silicon carbide in pressure transducers is illustrated in U.S. Pat. No. 5,165,283 entitled “High Temperature Transducers and Methods of Fabricating the Same Employing Silicon Carbide” issued on Nov. 24, 1992 to A. D Kurtz et al and assigned to the assignee herein.
In further pursuing the development of such devices, the assignee herein has disclosed other silicon carbide pressure transducers capable of operating at high temperatures, see U.S. Pat. No. 5,549,006 entitled “Temperature Compensated Silicon Carbide Pressure Transducer and Methods for Making the Same” issued on Aug. 27, 1996. As one can ascertain, there are many ways of fabricating silicon carbide transducers and such techniques are further illustrated in U.S. Pat. No. 6,034,001 entitled “Methods for Etching of Silicon Carbide Semiconductor Using Selective Etchings of Different Conductivity Types” issued on Mar. 7, 2000 to A. D. Kurtz et al and assigned to the assignee herein. In that patent, there are disclosed methods for selective conductive etching of a silicon carbide semiconductor utilizing hydrofluoric acid (HF) as well as other techniques.
U.S. Pat. No. 6,327,911 entitled “High Temperature Pressure Transducer Fabricated from Beta Silicon Carbide”, U.S. Pat. No. 6,689,669 entitled “High Temperature Sensors Utilizing Doping Controlled, Dielectrically Isolated Beta Silicon Carbide (SiC) Sensing Elements On A Specifically Selected High Temperature Force Collecting Membrane”, U.S. Pat. No. 6,900,108 entitled “High Temperature Sensors Utilizing Doping Controlled, Dielectrically Isolated Beta Silicon Carbide (SiC) Sensing Elements On A Specifically Selected High Temperature Force Collecting Membrane” issued on May 31, 2005 to A. D. Kurtz et al, and U.S. Pat. No. 6,691,581 entitled “Pressure Transducer Fabricated From Beta Silicon Carbide” issued on Feb. 17, 2004 to A. D. Kurtz et al further disclose high temperature pressure transducers using silicon carbide or beta silicon carbide sensing elements. Pursuant to the above, one can see that there are many methods of fabricating silicon carbide transducers. These methods further include fabricating dielectrically isolated transducers utilizing various techniques.
As is known and indicated in the above-noted patents, such techniques enable one to provide silicon carbide transducers capable of operating at temperatures well in excess of 600° C.
For example, reference is made to U.S. Pat. No. 6,691,581 entitled “Pressure Transducer Fabricated from Beta Silicon Carbide” issued on Feb. 17, 2004 to A. D. Kurtz et al. This patent shows a method for fabricating a dielectrically isolated silicon carbide high pressure transducer. The method comprises applying a layer of beta silicon carbide of a first conductivity on a first substrate of silicon. A layer of beta silicon carbide of a second conductivity is then formed on a second substrate. A layer of silicon is sputtered, evaporated or otherwise formed on the silicon carbide surfaces of each of the substrates of the beta silicon carbide. The sputtered silicon layer on each substrate is then completely oxidized forming a layer of silicon dioxide from the silicon. The first and second substrates are fusion bonded together along the oxide layers of the first and second substrates, with the oxide layer providing dielectric isolation. As one can ascertain, this is a desirable method, but requires extensive processing steps. See U.S. Pat. No. 6,900,108 entitled “High Temperature Sensors Utilizing Doping Controlled, Dielectrically Isolated Beta Silicon Carbide Sensing Elements On A Specifically Selected High Temperature Force Collecting Membrane” issued on May 31, 2005 to A. D. Kurtz et al and assigned to the assignee herein. That patent shows structures and techniques to fabricate high temperature devices, such as piezoresistive sensors. One employs crystalline doped silicon carbide which is dielectrically isolated from the substrate. The devices are formed by processes that include bonding a pattern wafer to a substrate wafer, selective oxidation or removal of the un-doped silicon and conversion of doped silicon to crystalline silicon carbide. The level of doping and the crystalline structure of the silicon carbide can be selected according to desired properties. This technique involves converting a doped layer of silicon to crystalline silicon carbide. It also produces a high temperature transducer according to the processes described therein.
U.S. Pat. No. 6,689,669 issued on Feb. 10, 2004 and entitled “High Temperature Sensors Utilizing Doping Controlled Dielectrically Isolated Beta Silicon Carbide Sensing Elements On A Specifically Selected High Temperature Force Collecting Membrane” shows the formation of a high temperature transducer where a first substrate wafer of silicon carbide has a layer of silicon dioxide formed on a surface. A second wafer of silicon is then employed. The second wafer is doped to enable conversion of the doped surface into a layer of silicon carbide of a targeted resistivity. The first wafer is bonded to the second wafer with a layer of silicon carbide of the first substrate contacting the doped surface of the second wafer where the bond is formed between the doped surface and the silicon dioxide layer. One then removes all of the silicon from the pattern wafer, thus leaving the doped silicon layer bonded to the oxide layer secured to the silicon carbide wafer, and then converts the doped silicon surface to silicon carbide to provide a surface of silicon carbide capable of being processed into a semiconductor device. As one can ascertain, that patent shows many necessary steps for processing silicon and silicon carbide. Reference again is made to U.S. Pat. No. 6,327,911 entitled “High Temperature Pressure Transducer Fabricated from Beta Silicon Carbide” issued on Dec. 11, 2001 to A. D. Kurtz et al. That patent shows a high temperature pressure transducer which employs dielectrically isolated beta silicon carbide pressure sensing elements situated on a diaphragm, and also fabricated from beta silicon carbide.
The dielectrically isolated pressure sensing elements are formed on the diaphragm in a method which employs two separately fabricated wafers that are later bonded together. This again is an analogous technique but requires many more steps and is a more expensive operation. That patent further discloses a technique that requires first depositing a diaphragm layer of beta silicon carbide on the surface of a first substrate which is a silicon substrate. One then forms a dielectric layer over the diaphragm layer of the beta silicon carbide. A piezoresistive Wheatstone bridge is then fabricated on a second substrate, the piezo sensing element comprising beta silicon carbide. One mounts the sensing element to the dielectric layer wherein at least one piezoresistive sensing element is dielectrically isolated from the diaphragm and then removes a portion of the second substrate so that the piezoresistive sensing element is exposed and one then removes a portion of the first substrate to form a flexible diaphragm.
The process used according to an aspect of the present invention is a simpler process to produce high temperature devices from silicon carbide. An improved silicon carbide pressure transducer and a technique for fabricating the same is described herein. Such a device is capable of high temperature operation is economical to fabricate and may be manufactured to be of extremely small dimensions.